Lectures

We break down the contract for creating a resource via POST /api/v1/tasks: how to decode JSON strictly and safely ( json.Decoder + DisallowUnknownFields() + body size limit), how to validate and normalize fields via a typed ValidationError, and how to produce a correct HTTP response: 201 Created with a Location header and a JSON body, or 400 Bad Request in a unified error envelope.

We break down the list endpoint contract for GET /api/v1/tasks: why the response is better wrapped into an object { "tasks" : [...] }, how to correctly read query parameters via url.Values, validate done/ sort, filter without unnecessary allocations, and make the output deterministic: sorting by ID by default, and Title + tie-break by ID when sort=title. As a finale, we assemble the handler pipeline parse → list → filter → sort → encode on net/http.

We break down how to design an endpoint for marking a task as done in a clean way: what to choose — PATCH or POST, how to ensure idempotency, why for success it is often better to return 204 No Content rather than 200, and how to implement it in Go 1.22+ using ServeMux and PathValue. At the end: common mistakes and how to avoid them.

We break down why DELETE is not “just delete,” but a contract that matters for retries and automation: what idempotency is (the effect matters, not the response), which status to choose on success ( 204 No Content), and what to do if the resource does not exist ( 404 Not Found). We implement DELETE /api/v1/tasks/{id} the Go way: a single {id} parsing approach via parseID, a domain error ErrNotFound, and mapping via errors.Is plus a shared writeError.

We break down how to turn errors into part of the API contract: introduce a unified JSON error envelope, define error classes ( validation/ not_found/ internal), set up a centralized policy via mapError and writeError, and connect domain errors ( ErrNotFound, ValidationError) to statuses 400/ 404/ 500, avoiding internal detail leaks and using errors.Is/ errors.As.